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Desalination technologies are gaining in popularity nowadays, as fresh water scarcity has been widely recognized as a global crisis [1]. Amongst the mature and developed desalination technologies, electro-dialysis (ED) is most suitable to treat feed streams with a concentration much lower than seawater [2]. While reverse osmosis (RO) is believed to be a mature technology for seawater desalination, as it operates towards a theoretical optimum concerning energy consumption [3], it cannot be used to desalinate feed streams with high salinities, such as seawater RO brine and certain waste waters produced by the oil and gas industry [4]. This is because of the high hydraulic pressure requirement of such feed streams. Thermal distillation technologies including multi-stage flash (MSF), multi-effect distillation (MED), vapor compression (VC) or membrane distillation (MD) consume a large amount of energy, though they can be used to desalinate brines. Therefore, it is desirable to develop alternative desalination technologies that can efficiently treat highly concentrated brines.
Forward osmosis (FO) has shown promise [5] in this regard. The permeation of water through a membrane is an automatic process driven by the osmotic pressure difference, or rather chemical potential gradient. In addition, the fouling in FO is significantly less and much easier to clean than RO because FO operates at much lower hydraulic pressures [6]. However, the product of an FO process alone is a diluted draw solution, which actually has a higher osmotic pressure than the feed solution. Therefore, the success of FO as a viable desalination technology calls for the discovery and development of more suitable draw solutes that can be regenerated efficiently and cost effectively. Although many inorganic salts [7] and organic compounds [5] have been studied as draw solutes that show a sufficiently high drawing ability against a feed with salinity equal to or even higher than seawater, their regeneration actually consumes more electrical energy than RO [8]. The entire process is therefore not economically viable. In fact, all non-responsive or non-regenerable draw solutes, including polyelectrolytes, zwitterionic compounds, quantum dots, organic salts and hydroacid complexes [9-13] face the same formidable challenge discussed above. Therefore, exploring ‘smart’ and regenerable draw solutes that can substantially reduce the osmotic pressure of the diluted draw solute after FO process via a certain stimulus or reaction have recently become a focused study topic concerning FO technology. Magnetic nanoparticles [14] and inorganic salts (CuSO4 or MgSO4) that can be recovered by metathesis [15] have been investigated. The problems for magnetic nanoparticles are insufficient osmotic pressure and severe agglomeration [16], while metathesis regeneration of CuSO4 or MgSO4 is tedious and requires the use of large amounts of chemicals.
Another group of smart draw solutes are molecules including macromolecules with lower critical solution temperature (LCST). At temperatures lower than the LCST, draw solutes dissolve in water to make a homogeneous draw solution; while at temperatures higher than the LCST, the hydrophobic interaction between draw solutes dominates to engender phase separation. Modified polyethylenimine [17], ethylene glycol ethers [18] and thermally responsive ionic liquids [19] have been proven to be capable of seawater desalination at a bench scale. It is worth noting that these LCST-type regenerable draw solutes stem from a subtle balance between hydrophilicity and hydrophobicity. While higher hydrophobicity leads to lower LCST (e.g.,<60° C.) which favors the ease of draw solute recovery, higher hydrophilicity is desired to generate a higher drawing ability.
CO2 responsive organic amine compounds [20] and dual responsive polymers reported recently are also promising draw solutes. These compounds/polymers become charged electrolytes or polyelectrolytes after protonation by CO2 and produce a high osmotic pressure in FO processes. Advantageously, the compounds/polymers revert back to uncharged natural or thermally responsive states via removal of CO2 to facilitate the regeneration process. The use of dual responsive polymers as draw solutes [21] further reduced the draw solute back diffusion and mitigated the issue of membrane damage by draw solutes based on low molecular organic amines.
Another effective thermolytically regenerable draw solute is based on ammonia-carbon dioxide system [22-24]. Thermolytic salts including ammonium bicarbonate, ammonium carbonate and ammonium carbamate are highly soluble, can generate very high osmotic pressures and can thermally decompose into CO2 and NH3 gases, which are then recombined to form the original draw solutes in the regeneration process.
Although the thermally regenerable draw solutes (thermolytic and LCST-type) discussed above may not enable FO to consume less energy than RO, they can lower the energy cost by using cheaper low grade thermal energy instead of electrical energy in the regeneration process [25]. For example, using a thermally responsive ionic liquid as draw solute [19], the theoretical electrical energy consumption for seawater desalination is only a fraction (16%) of that for RO. Low grade thermal energy can be used to make up the rest of the energy required for the separation, leading to potentially significant saving of energy cost and reduction of carbon footprint.